Generally, nylon block copolymers may be alternating blocks of polyamide segments and other segments such as segments of elastomeric polymers such as polyethers, polyesters, hydrocarbons or polysiloxanes. These nylon block copolymers are generally prepared by copolymerizing a lactam monomer in the present of the elastomeric polymers component. A more detailed discussion of the structure and method of preparing particular types of nylon block copolymers can be found in U.S. Pat. No. 4,031,164.
The polyamide segments and elastomeric polymer segments of the nylon block copolymers each contribute to the respective properties of the final polymer. In order to obtain high modulus materials, polyamide segments of higher molecular weight and/or higher weight percent can be employed. Alternatively, greater tensile elongation and impact properties, as well as lower surface hardness, may be obtained by using higher percents of and/or higher molecular weight elastomeric polymer component.
U.S. Pat. No. 4,671,355 appears to be one of the first disclosures of a crosslinked nylon block copolymer, chemically, through the use of polyfunctional amine compounds. That is, the crosslinked nylon block copolymers are prepared by a reaction scheme in which polyfunctional amines act as crosslinking agents. More specifically, a crosslinked material was reportedly synethesized by reacting an acyl lactam fucntionalized material with the polyfunctional amine to prepare crosslinked acyl lactam materials which were then concurrently or subsequently reacted with lactam monomer in the presence of a lactam polymerization catalyst to form the crosslinked nylon block copolymer material. It was reported therein that by chemically crosslinking, it was discovered that the overall properties of the final polymer could be varied even if one maintains the molecular weight and weight percent of the elastomeric polymer component.
While the above chemical method of crosslinking a nylon block copolymer has been reported, no reports exist concerning the development of a nylon block copolymer by a more convenient method such as irradiation. The closest attempts in this regard can be found, for example, in Plast. Massy, 1993, No. 2, pp 35-37, which contains a paper entitled "Production and Properties of Crosslinked Compositions of Aliphatic Nylons". According to the abstract, a study was conducted on the process of radiation crosslinking of an aliphatic polyamides (i.e., not a nylon block copolymers) and an assessment is made of the properties and network compositions obtained. The materials studied were nylon-6, nylon-6,6 and nylon-12. The polyfunctional monomers employed to accelerate crosslinking were triallyl cyanurate and triallyl isocyanurate. Mechanical data is supplied.
Similarly, in the Chinese Journal of Polymer Science, Vol. 7, No. 1, there is a paper entitled "Characterization of Irradiated Crystalline Polymer-Isothermal Crystallization Kinetics of Radiation Induced Crosslinked Polyamide 1010". As disclosed therein, after irradiation, the service temperature of the resin is raised to about 240.degree. C. In addition, network formation is said to greatly change the crystallization behavior of the otherwise crystalline polyamide material.
Finally, it is worth noting that various other disclosures have been uncovered, which recite thermoset (or crosslinked) polyamide resins, but again, no mention or suggestion of irradiation crosslinking of a nylon block copolymer is described. For example, in U.S. Pat. No. 5,198,551 entitled "Polyamide Thermosets" there is disclosed what is termed curable polyamide monomers, curable liquid crystal polyamide monomers and thermoset compositions prepared therefrom. The thermoset polyamides so prepared all contained highly aromatic type structure. Similarly, in U.S. Pat. No. 5,3154,011, which is a divisional of the '551 Patent, there is again described curable polyamide monomer systems, which monomers represent highly aromatic type functionality.
The fact that there have been no reports concerning the development of a convenient route for the preparation of a crosslinked nylon block copolymer is underscored when reference is made to U.S. Pat. No. 5,584,821, which discloses an angiographic catheter which has a relatively stiff though flexible shaft and a soft tip. The soft tip consists primarily of a tungsten loaded polyether block amide (PEBA) copolymer surrounded by two thin PEBA layers. This three ply radiopaque tip is bonded to a PEBA shaft. The shaft is reinforced either by an inner nylon ply or by metal braiding.
In other words, pursuant to the teachings of U.S. Pat. No. 5,584,821 when it comes to the production of a soft tip catheter with a relatively stiffer body, the teachings therein emphasize that the stiffer body portion relies upon the use of a metal braided reinforced PEBA copolymer or a co-extruded two ply wall consisting of nylon and PEBA copolymer. That being the case, it becomes clear that inasmuch as PEBA type copolymers are widely used in catheter type applications, it would serve a long-standing need if one could conveniently produce a more rigid and toughened PEBA catheter, without the need for the structural modifications emphasized in the prior art.
Accordingly, it is an object of this invention to prepare a crosslinked nylon block copolymer, wherein said polymer is conveniently crosslinked by the process of irradiation or other high energy source, wherein such crosslinked nylon block copolymer has particular utility as a component of a medical catheter product.
More specifically, it is object of the present invention to prepare a crosslinked nylon block copolymer elastomeric formulation, via irradiation techniques, wherein the elastomeric composition, subsequent to crosslinking, exhibits improvement in properties such as mechanical strength, heat resistance, and hardness, and in particular, the crosslinked material so produced demonstrates elongational behavior when exposed to elevated temperatures under conditions of constant stress.
Furthermore, it is an object of the present invention to crosslink nylon block copolymer systems, wherein such crosslinking improves the overall elastomeric toughness of the block copolymer, thereby providing what can be termed a much more durable nylon block copolymer product for a variety of miscellaneous applications in the medical industry.